The analogy for the Aurors as simple detectives is a disingenuous at best: Aurors are an elite, specialized unit and are thus more well rounded/dangerous than a normal investigator. Their training takes longer, and their entry requirements are higher (5 NEWTs + 3 year training vs 5 OWLS and being over 17). They have Stealth and Concealment training, and are able to use Memory Charms (Shacklebolt used one on Marietta Edgecombe). While Hit Wizards might be akin to SWAT (a fast response team for the Magical Law Enforcement Squad), Aurors are more like CIA/Delta Force personnel, managing Dark Wizards (domestic terrorists) and those dangling with the Dark Arts. But their intelligence work and their investigative skills shouldn't detract from their presence and ability on the field.
While Hit Wizards arrested Sirius, virtually all of the other Death Eaters were hunted and took down by the Aurors. Moreover they were able to use the Unforgivable Curses under Crouch's Administration (reminiscent of James Bond/MI6 "license to kill") and during the First Wizarding War Auror squads were routinely fighting and winning against Giants. If you don't think Aurors take action on the frontlines, just look at Moody. Who's more dangerous, John McClane (police) or Jason Bourne (secret services)? Hard to tell, but the latter can find far more numerous and creative ways to fuck you up than the former ever will.